The increase in demand for portable devices and technology scaling are driving down the supply voltages of digital circuits. A voltage reference generator is used in many integrated circuits (ICs). The bandgap reference generator which can operate from a 1V supply is widely used, for example, in DRAM and flash memories. A bandgap voltage reference should be insensitive to temperature, power supply and load variations.
One principle of operation of bandgap circuits relies on two groups of diode-connected bipolar junction transistors (BJT) running at different emitter current densities. By canceling the negative temperature dependence of the PN junctions in one group of transistors with the positive temperature dependence from a PTAT (proportional-to-absolute-temperature) circuit which includes the other group of transistors, a fixed DC voltage output, Vref, which doesn't substantially change with temperature is generated. This reference voltage is typically 1.26 volts, which is approximately equal to the bandgap voltage of silicon.
Recent IC designs sometimes require sub-1 volt operation regions. Additionally, for integrated circuits used in thermal sensors or three-dimensional (3-D) IC applications, for example, it is desirable to have a very small temperature coefficient bandgap reference voltage in order to sense temperature variations. Some bandgap reference circuits, however, can become unstable or lose accuracy as a result of variation in input offset voltages applied to an operational amplifier of the bandgap reference circuit and/or current mirror mismatch effects. However, at low input offset voltages applied to the operational amplifier, the current mirror mismatch effect will dominate and can degrade the accuracy and performance of such bandgap reference circuits.